Acoustic lens

ABSTRACT

An acoustic lens for dispersing sound waves transmitted therethrough comprising a biconcave, plano-concave or negative meniscus shaped, open celled plastic foam. Lens is preferably biconcave and formed from an explosively reticulated polyurethane foam, and is particularly useful in dispersing high frequency sound waves from loudspeaker systems to provide balanced response.

finite States Patent [191 Long [451 May 22,1973

[54] ACOUSTIC LENS [75] Inventor: Edward M. Long, Oakland, Calif.

[73] Assignee: Ampex Corporation, Redwood City,

Calif.

22 Filed: Mar. 10,1971

211 App]. No.: 122,887

[52] US. Cl ..340/8 L, 18l/.5 R [51 1 Int. Cl ..H04b 13/00, H04r 7/02 [58] Field of Search ..340/8 L, 8 MM, 8 R;

[56] References Cited UNITED STATES PATENTS 8/1948 Gillespie ....340/8 L 8/1966 Massa ..340/8 L Primary Examiner-Benjamin A. Borchelt Assistant Examiner-J. V. Doramus AttorneyAnderson, Luedeka, Fitch, Even & Tabin 57 1 ABSTRACT An acoustic lens for dispersing sound waves transmitted therethrough comprising a biconcave, planoconcave or negative meniscus shaped, open celled plastic foam. Lens is preferably biconcave and formed from an explosively reticulated polyurethane foam, and is particularly useful in dispersing high frequency sound waves from loudspeaker systems to provide balanced response.

11 Claims, 5 Drawing Figures PATENTEU HAY 2 2 I975 INVENTOQ [award M lava MM, 314% 414, 6/54, mQ

ATTYS.

ACOUSTIC LENS This invention relates to acoustic dispersion lenses, and more particularly to open cell plastic foam devices suitable for dispersing sound waves.

It is known that as the frequency of sound waves radiated from a loudspeaker increases, the radiated sound energy tends to be increasingly oriented in a direction along the axis of the loudspeaker. Accordingly, while low frequency bass tones tend to disperse more uniformly, high frequency treble tones tend to propagate in a straight line directly out from the front of the loudspeaker; in multirange speaker systems the acoustic radiation from treble speakers tends to be more axially oriented than that from bass speakers.

This directional characteristic of the higher frequency range of the response from loudspeaker systems is particularly undesirable for the reproduction of music and other sounds where acoustical accuracy is desired. When the high frequency output is more directionally oriented than the low frequency output, the acoustic reproduction will be unbalanced, with excess high frequency response along loudspeaker axes and a high frequency deficiency at angles off speaker axes.

The high frequency dispersion, and consequently the acoustic balance, of loudspeaker systems has been improved by means of conventional acoustic dispersion lenses. However, such conventional dispersion lenses are comprised of relatively complex arrays of metal strips, perforated sheets, or slanted sheets, and their success and general use has been limited.

It is an object of this invention to provide an acoustic lens suitable for dispersing sound waves. It is a further object of this invention to provide a relatively inexpensive acoustic lens suitable for dispersing high frequency sound waves in order to improve the acoustic balance of loudspeaker systems.

These and other objects of the invention are more particularly set forth in the following detailed description and in the accompanying drawings of which FIG. 1 is a perspective front view of an acoustic dispersion lens;

FIG. 2 is a cross sectional side view of the dispersion lens of FIG. 1 taken along line 2-2;

FIGS. 3 and 4 are graphic representations of the increase in acoustic dispersion resulting from the lens of FIG. 1, and

FIG. 5 is a cross sectional side view of another acoustic lens of the present invention.

Generally, the present invention is directed to an acoustic dispersing lens for dispersing sound waves which are transmitted through the lens. The body of the lens is constructed from an open-celled plastic foam and has a generally biconcave, planoconcave or negative meniscus shape. The lens has an entrance face for sound waves to enter the lens and an exit face from which sound waves which have entered the lens by means of the entrance face may leave the lens in a more dispersed propagational pattern. At least one of the entrance or exit faces has a concave shape. It is preferred that the lens have a biconcave or planoconcave shape in which either both the entrance face and the exit face have a concave shape, or in which one of the entrance or exit faces is planar and the other face has a concave shape. For designs to develop maximum acoustic dispersion for a given plastic foam, bi-concave lens shapes in which both the entrance and exit faces both have concave shapes, are particularly preferred.

Preferably the body of the lens is made of those opencelled plastic foams having about 30 or more pores (or cells) per lineal inch. Particularly preferred are those open-celled plastic foams which have between about 30 and about pores (or cells) per lineal inch. It is also particularly desirable that such open-celled plastic foams have a density of between about 1 and about 3 pounds per cubic foot. Explosively reticulated rigid polyurethane foams are particularly preferred and have been found to provide acoustic lenses of the present invention having excellent acoustic dispersing properties.

With reference to the embodiments of FIGS. 1 and 2 of the drawings, the acoustic dispersion lens 10 there shown comprises a body 12 formed from an opencelled plastic foam. The body 12 has radially symmetrical entrance and exit faces 14 and 16, respectively, which have the same axis of symmetry 17. A circumferential edge 18, which is a partial toroidal surface section and which is also radially symmetrical with respect to axis 17, flares outwardly from its rear boundary at its intersection 20 with the entrance face 14 to its forward boundary at its intersection 22 with the exit face 16.

.Adjacent to and surrounding the circumferential edge 18 of the lens 10 is a lens holder 24 having rim portions 26 which project directly outwardly at the exit face 16 and extend directly inwardly at the entrance face 14. The lens holder 24 is fabricated in any suitable manner and from any suitable material, such as by injection molding from polystyrene or stamping from sheet metal.

The entrance face 14, into which the acoustic radiation to be dispersed by the lens 10 is directed, is comprised of a spherically concave rear surface 28 having a radius of curvature rr emanating from a point 34 located along the axis 17, and a rear planar rim 30 which surrounds the concave rear surface 28 and extends from its intersection 32 with that surface to its intersection 20 with the circumferential edge 18. Accordingly, both of the intersections 20 and 32 of the rear planar rim 30, respectively with the concave rear surface 28 and the circumferential edge 18, are circular and lie in a plane perpendicular to axis 17. Accordingly, the diameter of the circular intersection of the rear planar rim 30 with the circumferential edge defines the physical size of the entrance face 14, while the diameter of the intersection 32 with the concave surface 28 determines the functional size of the entrance face 14.

The exit face 16 similarly is comprised of a spherically concave front surface 34 having a radius of curvature R-R' emanating from a point 36 located along the axis 17, and a front planar rim 38 which is perpendicular to axis 17 and surrounds the concave front surface 34 and which has a circular intersection 35 with the concave front surface 34. Also part of the exit face, and surrounding the front planar rim 38, is a beveled surface 40, which flares in a direction opposite that of, and extends to, the circumferential edge 18. Accordingly, the beveled surface 40 intersects with both the front planar rim 38 and the circumferential edge 18 along circles 42 and 22, respectively, which lie in separate planes each perpendicular to axis 17. v

The degree of acoustic dispersion accomplished by means of the lens 10 is related to the radii of curvature r-r' and R-R' of the concave surfaces 28 and 34 of the entrance and exit faces 14 and 16; generally, the degree of acoustic dispersion is inversely related to the radii of curvature of the concave surfaces 28 and 34 of the lens 10, so that decreasing the radii of curvature (e.g., increasing the degree of curvature of the surfaces) will result in an increase in the degree of acoustic dispersion achieved by the lens.

in operation, the acoustic radiation to be dispersed is transmitted through the lens from the entrance face 14 to the exit face 16. The size of the entrance face, and particularly the functional size of the entrance face as related to the size of the concave rear surface 28, will vary depending upon the application and the size of the acoustic wavefront to be dispersed by the lens. For example, the embodiment of FIGS. 1 and 2 is particularly adapted for placement of the entrance face 14 closely adjacent to the speaker cone of a loudspeaker such as the high frequency speaker or tweeter of a multirange loudspeaker system, and in such circumstances the size of the concave rear surface 28 of the entrance face 14 should preferably correspond approximately to the size of the speaker cone.

The concave front surface 34 of the exit face 16 should preferably be larger than the concave rear surface 28 of the entrance face 14 in order to most effectively accommodate and disperse the acoustic radiation transmitted through the lens. The larger size of the convex surface of the exit face becomes increasingly desirable as the radius of curvature r-r' of the concave rear surface of the entrance face decreases, and the concave front surface of the exit face will ordinarily be increasingly larger than that of the entrance face (or that part of the entrance face into which the acoustic radiation to be dispersed is transmitted) as the radius of curvature of the concave rear surface of the entrance face decreases. It should be noted that when shorter radii of curvature are employed for the concave rear surface of the entrance face, that shorter radii of curvature will also ordinarily be employed for that of the exit face.

Accordingly, it is preferred that the diameter of the intersection 35 of the front planar rim 38 with the concave front surface 34 of the exit face 16 be increasingly larger than the diameter of the intersection 32 of the rear planar rim with the concave rear surface 28 of the entrance face 14 as the radius of curvature r-r' of the latter surface decreases. It should also be noted that while the thickness 46 of the lens 10 along the axis 17 is preferably relatively small (i.e., less than one fifth of the diameter of the concave rear surface 28 of the entrance face 14), increasing this thickness also tends to increase the desirability of having a larger concave surface on the exit face than on the entrance face. Generally, when employed to disperse sound waves from high frequency tweeter" loudspeakers, the dispersing lens will have an entrance face 14 with a concave rear surface 28 having a diameter of between about 1% inches and about 3% inches and a radius of curvature r-r of between about 1 inch and about 3 inches, and an exit face 16 having a concave front surface 34 having a diameter of between about 2 inches and about 5 inches and a radius of curvature R-R between about 1% inches and about 4 inches. The maximum possible diameter of such a spherical surface is twice the radius of curvature of the surface, achieved when the point from which the radius of curvature emanates lies in the plane of the planar rim which intersects with that surface. Accordingly, the solid angle swept out by the spherical surfaces from the point from which their radii of curvature emanate will ordinarily be less than 211' steradians. However, special situations may exist, such as the sound source being relatively small with respect to the entrance face and located relatively close to that face, where it might be desirable to employ surfaces having somewhat larger solid angles. For such use, an effective open-celled foam is an explosively reticulated polyester polyurethane foam having between about 30 and about pores per lineal inch and a density of between about 2 and about 5 pounds per cubic foot.

The following examples demonstrate the dispersion of high frequency acoustic radiation which may be achieved employing an open-celled plastic foam acoustic lens of the present invention.

EXAMPLE I An acoustic lens of the design shown in FIGS. 1 and 2 of the drawings is shaped from an open-celled explosively reticulated polyester polyurethane foam having a high degree of openness, which has 60 pores per lineal inch. The foam has a density of 2 pounds per cubic foot. The entrance face of the lens has a spherically concave rear surface with a radius of curvature of 1% inches, which is contiguously surrounded by a rear planar rim with an inside diameter of 2 inches and an outside diameter of 2% inches. The exit face has a spherically concave front surface with a radius of curvature of 2% inches which is contiguously surrounded by a front planar rim with an inside diameter of 2-25/32 inches and an outside diameter of 4% inches. The thickness of the lens along the axis is one-eighth inch, and the distance between the planes of the front planar rim and the rear planar rim is 1% inches. The front planar rim is surrounded by a bevel which flares outwardly toward the entrance face to intersect with the toroidal circumferential edge which itself flares outwardly in the opposite direction from the outside edge of the rear planar rim to its intersection with the beveled edge. The outside diameter of the beveled edge, which is the widest part of the lens, is 4% inches, and the distance between the plane of the front planar rim and the plane of the intersection of the bevel with the circumferential edge is 5/32 inch. The lens is inserted in a high impact polystyrene lens holder which surrounds the circumferential edge of the lens.

A high frequency tweeter loudspeaker having a cone diameter of 2% inches is mounted in the center of an anechoic chamber so that the propagation pattern of high frequency sound radiated from it without employing the dispersion lens may be measured. The speaker is supplied with an input signal at a frequency of 7,500 Hz., and the radial distribution of acoustic output of the speaker is then measured at a distance of 18 from the apex of the speaker cone along a are which is bisected by the axis of the speaker. The measured radial distribution of acoustic output from the speaker at a frequency of 7,500 Hz. without the lens is represented by the dashed curve of FIG. 3.

The speaker is then supplied with an input signal at a frequency of 13,000 Hz. and the radial distribution of acoustic output of the speaker is measured in the same manner as before. The measured radial distribution of acoustic output from the speaker at a frequency of 13,000 Hz. without the lens is represented by the dashed curve of FIG. 4.

The entrance face of the acoustic lens is then placed directly in front of, and adjacent to, the high frequency loudspeaker so that high frequency sound waves from the speaker will enter the entrance face of the lens, be transmitted therethrough, and emerge from the exit face. The radial distribution of the acoustic output from the speaker at both 7,500 Hz. and 13,000 Hz. is then measured as before; the solid curve of FIG. 3 represents the measured radial distribution of acoustic output at 7,500 Hz. with the lens, and the solid curve of FIG. 4 represents the measured radial distribution of acoustic output at 1,300 Hz. with the lens.

A comparison of the radial acoustic output distributions at both 7,500 Hz. and 13,000 Hz. without the lens, with the acoustic output distribution obtained by employing the lens, reveals a significant increase in acoustic dispersion resulting in higher acoustic output at angles off the speaker axis; accordingly, the propagational pattern of the sound waves from the loudspeaker is more dispersed upon emerging from the exit face of the lens.

EXAMPLE II Two acoustic lenses like that described in Example I are mounted in front of the high frequency tweeters of two enclosed loudspeaker systems designed for home stereo music listening, each comprising an air suspension type 8" woofer and a cup chamber tweeter. Music is played through the speaker system in room volumes of 700 cubic feet to 3,000 cubic feet having medium to live acoustics (reverberation time, 0.8 to 1.2 seconds). The high frequency-low frequency balance, dispersion and acoustic accuracy are improved over an identical speaker system which does not employ the acoustic lenses.

Illustrated in FIG. 5 is a plano-convex embodiment of the present invention. In this embodiment, the dispersing lens 50 comprises a body 52 formed from an opencelled polyurethane foam having a density of 2 pounds per cubic foot and 30 pores per lineal inch.

Like the biconcave lens of FIGS. 1 and 2, the planeconcave lens 50 of FIG. 5 is radially symmetrical with circular faces, and is shown only in cross section through the axis. The circular entrance face 54 is planar and perpendicular to the axis 56 of the lens 50. Separating the entrance face 54 from the circular exit face 58 is a frustro-conico beveled circumferential edge 60 which flares outwardly from the entrance face 54 to the exit face 58. The exit face itself has a spherically concave central portion 62 which has a radius of curvature AA emanating from a point 64 located along the axis 56. This radius of curvature AA will normally be less than about 4 inches for open cell plastic foam lenses for high frequency loudspeakers, and preferably between about 2 inches and about 3% inches. Contiguously surrounding the spherically concave central portion of the exit face is a peripheral portion 66 of the exit face 58 which is also radially symmetrical with respect to the axis 56, and is a section of a toroidal surface which is curved in a direction opposite that of the central concave portion 62, but which intersects smoothly with that surface along circle 68 which separates the central portion 62 of the exit face 58 from the peripheral portion 66 and which is defined by the inflection of curvature between those two surfaces. The thickness 70 of the lens 50 along the axis 56 is preferably relatively small with respect to the diameter of the lens. Planoconcave lenses having a concave entrance face and a plane exit face may also be employed.

Although the preferred embodiments illustrated in FIGS. 1, 2 and 5 have both smooth surfaces and spherical or other circularly symmetrical faces, other designs having a biconcave plane-concave, or negative meniscus shape are also useful. For example, non-spherical, concave lens faces, such as a concave, prolate ellipsoidal surface having its axis of revolution perpendicular to the axis of the lens, might be employed for certain applications. A lens having such an ellipsoidal surface would tend to disperse the sound waves from the loudspeaker to a greater extent in a direction perpendicular to the axis of revolution of the surface than in a direction parallel to the axis of revolution of the surface. Lenses having such directionally selective dispersing properties may be useful for individual speakers, for example, where it is desired to have more horizontal dispersion than vertical dispersion, as well as for arrays of speakers such as a line of speakers in which it is desired to uniformly disperse the cumulative output of such speakers. A loudspeaker which does not have a relatively uniform power distribution across its face, but rather develops concentrated acoustic radiation from the central portion or apex of the speaker which is highly directional along the speaker axis, may best em-.

ploy dispersing lenses having concave hyperbolic or cone-shaped surfaces, which surfaces accordingly have a relatively greater dispersive effect near the axis of the lens than near the periphery.

Furthermore, although it is preferred that the lens surfaces be smooth, smooth surfaces may be approximated. Thus, while it is also preferred that the body of the lens be comprised of a single, unitary piece of opencelled plastic foam, by assembling laminae having suitably graduated holes or recesses, a dispersing lens having, for example, a flat entrance face and a concave exit face 68 with a stepped surface may be provided, which will disperse sound waves.

It will be noted that acoustic lenses of the present invention may be stacked to form multiple lens arrays or elements, each of which lenses remains within the scope of the present invention. For example, two plano-concave lenses of suitable size may be aligned so that they abut and have the same axis, and when mechanically fastened or glued together form a composite acoustic lens having two internal concave faces and two external concave faces. Similarly, two planoconcave lenses may be stacked to form, for example, a biconcave lens having two internal concave surfaces and two external plane surfaces.

The acoustic dispersion lens may be fabricated by any method which will provide the desired shape of the open celled foam. Examples of useful methods include machining by means of conventional foam cutting equipment, and hot wire cutting. Machining of the preformed open-celled foam to the desired shape is a versatile and preferred fabrication method. Rigid plastic foams, particularly rigid polyurethane foams, are readily machined into the desired shape. A hot blade or wire, such as a resistance-heated wire, may also be employed as a cutting means for shaping the surfaces of the dispersing lens.

Various of the features of the invention are set forth in the following claims. 0

What is claimed is:

1. An acoustic lens for dispersing sound waves transmitted therethrough, comprising a body formed from an open celled plastic foam having a density of between about 1 and about pounds per cubic foot and having about 30 or more pores per lineal inch, said body having an entrance face for sound waves to enter the lens and an exit face from which sound waves entering the entrance face may leave the lens in a more dispersed propagational pattern, wherein at least one of said faces is dispersively concave.

2. An acoustic dispersion lens according to claim 1 wherein the other of said faces is concave.

3. An acoustic dispersion lens according to claim 1 wherein the other of said faces is planar.

4. An acoustic lens according to claim 1 wherein at least one of said faces is spherically concave over at least a portion of the surface thereof.

5. An acoustic lens according to claim 2 wherein said open celled plastic foam is an explosively reticulated rigid polyurethane foam having a density of between about 1 and about 3 pounds per cubic foot and having between about 30 and about pores per lineal inch.

6. An acoustic lens according to claim 3 wherein said open celled plastic foam is an explosively reticulated rigid polyurethane foam having a density of between about 1 and about 3 pounds per cubic foot and having between about 30 and about 90 pores per lineal inch.

7. An acoustic lens according to claim 2 wherein the thickness of said lens along its axis is less than one-fifth the diameter of said entrance face.

8. An acoustic lens according to claim 3 wherein the thickness of said lens along its axis is less than one-fifth the diameter of said entrance face.

9. An acoustic lens according to claim 3 wherein said concave face is radially symmetrical.

10. An acoustic lens according to claim 2 wherein said concave faces are radially symmetrical.

11. An acoustic lens according to claim 9 wherein said radially symmetrical concave face has a central peripherally concave surface contiguously surrounded by a peripheral torroidal surface of opposite curvature. 

1. An acoustic lens for dispersing sound waves transmitted therethrough, comprising a body formed from an open celled plastic foam having a density of between about 1 and about 5 pounds per cubic foot and having about 30 or more pores per lineal inch, said body having an entrance face for sound waves to enter the lens and an exit face from which sound waves entering the entrance face may leave the lens in a more dispersed propagational pattern, wherein at least one of said faces is dispersively concave.
 2. An acoustic dispersion lens according to claim 1 wherein the other of said faces is concave.
 3. An acoustic dispersion lens according to claim 1 wherein the other of said faces is planar.
 4. An acoustic lens according to claim 1 wherein at least one of said faces is spherically concave over at least a portion of the surface thereof.
 5. An acoustic lens according to claim 2 wherein said open celled plastic foam is an explosively reticulated rigid polyurethane foam having a density of between about 1 and about 3 pounds per cubic foot and having between about 30 and about 90 pores per lineal inch.
 6. An acoustic lens according to claim 3 wherein said open celled plastic foam is an explosively reticulated rigid polyurethane foam having a density of between about 1 and about 3 pounds per cubic foot and having between about 30 and about 90 pores per lineal inch.
 7. An acoustic lens according to claim 2 wherein the thickness of said lens along its axis is less than one-fifth the diameter of said entrance face.
 8. An acoustic lens according to claim 3 wherein the thickness of said lens along its axis is less than one-fifth the diameter of said entrance face.
 9. An acoustic lens according to claim 3 wherein said concave face is radially symmetrical.
 10. An acoustic lEns according to claim 2 wherein said concave faces are radially symmetrical.
 11. An acoustic lens according to claim 9 wherein said radially symmetrical concave face has a central peripherally concave surface contiguously surrounded by a peripheral torroidal surface of opposite curvature. 